Biological degradation in a stream is a highly complex, “assembly-line” process. Most water-borne organics are consumed by microbes, but some substances may persist for long periods. Sources of refractory compounds may be industrial wastes, sewage plant discharges, land drainage, and even metabolic residues from microbial degradation of pollutants. Separation and identification of trace organics is an extremely difficult process requiring the most refined and sophisticated analytical methods. Increasing reuse of surface waters will require more surveillance and new approaches in water management.